Hydrogen is well known as one of the cleanest of available fuels because it can be reacted with oxygen to produce energy and water in hydrogen-consuming devices such as fuel cell and internal combustion engines, for example. Virtually no other reaction byproducts are produced in the exhaust, thereby avoiding well established environmental hazards associated with the use of petroleum based fuels.
Safe and efficient storage of hydrogen is essential for hydrogen-consuming applications. In particular, important factors involved in the development of mobile hydrogen-consuming applications include minimization of volume and weight of the hydrogen storage systems, while maximizing the safety thereof.
Conventional methods of storing hydrogen are generally either inadequate or impractical for wide-spread mobile consumer applications. For example, hydrogen can be stored in liquid form at very low temperatures, but at an unacceptably low efficiency. The energy consumed in liquefying hydrogen gas is about 40% of the energy available from the resulting hydrogen. Moreover, shelf-life is very limited; an ordinary tank filled with liquid hydrogen will become empty in about a week. These factors make liquid hydrogen impractical for most consumer applications.
An alternative is to store hydrogen under high pressure in cylinders. However, a 100 pound steel cylinder can only store about one pound of hydrogen at about 2200 psi, which translates into 1% by weight of hydrogen storage.(^) More expensive composite cylinders with special compressors can store hydrogen at higher pressures of about 4,500 psi to achieve a more favorable storage ratio of about 4% by weight. Although even higher pressures are possible, safety factors and the large amount of energy consumed in achieving such high pressures have compelled a search for alternative hydrogen storage technologies that are both safe and efficient.
In view of the above, there is a need for safe, efficient, effective methods and systems for storing and recovering hydrogen. In addition, there is a desire to minimize the overall system volume and weight.